Method and system for improving compressed image chroma information

ABSTRACT

Methods, systems, and computer programs for improving compressed image chroma information. In one aspect of the invention, a resolution for a red color component of a color video image is used that is higher than the resolution for a blue color component of the color video image. Another aspect includes utilizing a lower or higher value of a quantization parameter (QP) for one or more chroma channels as compared to the luminance channel. Another aspect is use of a logarithmic representation of a video image to benefit image coding. Another aspect uses more than two chroma channels to represent a video image.

TECHNICAL FIELD

[0001] This invention relates to video compression, and moreparticularly to methods, systems, and computer programs for improvingcompressed image chroma information in MPEG-like video compressionsystems.

BACKGROUND

[0002] MPEG Background

[0003] MPEG-2 and MPEG-4 are international video compression standardsdefining a video syntax that provides an efficient way to representimage sequences in the form of more compact coded data. The language ofthe coded bits is the “syntax.” For example, a few tokens can representan entire block of samples (e.g., 64 samples for MPEG-2). Both MPEGstandards also describe a decoding (reconstruction) process where thecoded bits are mapped from the compact representation into anapproximation of the original format of the image sequence. For example,a flag in the coded bitstream signals whether the following bits are tobe preceded with a prediction algorithm prior to being decoded with adiscrete cosine transform (DCT) algorithm. The algorithms comprising thedecoding process are regulated by the semantics defined by these MPEGstandards. This syntax can be applied to exploit common videocharacteristics such as spatial redundancy, temporal redundancy, uniformmotion, spatial masking, etc. In effect, these MPEG standards define aprogramming language as well as a data format. An MPEG decoder must beable to parse and decode an incoming data stream, but so long as thedata stream complies with the corresponding MPEG syntax, a wide varietyof possible data structures and compression techniques can be used(although technically this deviates from the standard since thesemantics are not conformant). It is also possible to carry the neededsemantics within an alternative syntax.

[0004] These MPEG standards use a variety of compression methods,including intraframe and interframe methods. In most video scenes, thebackground remains relatively stable while action takes place in theforeground. The background may move, but a great deal of the scene isredundant. These MPEG standards start compression by creating areference frame called an “intra” frame or “I frame”. I frames arecompressed without reference to other frames and thus contain an entireframe of video information. I frames provide entry points into a databitstream for random access, but can only be moderately compressed.Typically, the data representing I frames is placed in the bitstreamevery 12 to 15 frames (although it is also useful in some circumstancesto use much wider spacing between I frames). Thereafter, since only asmall portion of the frames that fall between the reference I frames aredifferent from the bracketing I frames, only the image differences arecaptured, compressed, and stored. Two types of frames are used for suchdifferences—predicted or P frames, and bi-directional interpolated or Bframes.

[0005] P frames generally are encoded with reference to a past frame(either an I frame or a previous P frame), and, in general, are used asa reference for subsequent P frames. P frames receive a fairly highamount of compression. B frames provide the highest amount ofcompression but require both a past and a future reference frame inorder to be encoded. Bi-directional frames are never used for referenceframes in standard compression technologies.

[0006] Macroblocks are regions of image pixels. For MPEG-2, a macroblockis a 16×16 pixel grouping of four 8×8 DCT blocks, together with onemotion vector for P frames, and one or two motion vectors for B frames.Macroblocks within P frames may be individually encoded using eitherintra-frame or inter-frame (predicted) coding. Macroblocks within Bframes may be individually encoded using intra-frame coding, forwardpredicted coding, backward predicted coding, or both forward andbackward (i.e., bi-directionally interpolated) predicted coding. Aslightly different but similar structure is used in MPEG-4 video coding.

[0007] After coding, an MPEG data bitstream comprises a sequence of I,P, and B frames. A sequence may consist of almost any pattern of I, P,and B frames (there are a few minor semantic restrictions on theirplacement). However, it is common in industrial practice to have a fixedpattern (e.g., IBBPBBPBBPBBPBB).

[0008] MPEG Color Space Representation

[0009] MPEG-1, MPEG-2, and MPEG-4 all utilize a Y, U, V color space forcompression. There is a choice of luminance equation, but a typicalconversion transformation between RGB (red-green-blue) to a YUVrepresentation is expressed as:

[0010] Y=0.59 G+0.29 R+0.12 B

[0011] U=R−Y

[0012] V=B−Y

[0013] The Y luminance factors for green range from 0.55 up to 0.75,depending upon the color system. The factors for red range from 0.2 to0.3, and the factors for blue range from 0.05 to 0.15.

[0014] This transformation can be cast as a matrix transformation, whichis a linear operator intended for use on linear signals. However, thissimple transformation is performed in MPEG 1, 2, and 4 in the non-linearvideo space, yielding various artifacts and problems.

[0015] It is typical in MPEG to reduce the resolution of the U and Vchroma channels to achieve higher compression. The most commonly usedreduction of resolution is to use half resolution both vertically andhorizontally. MPEG-2 supports full resolution chroma, as well as halfresolution horizontally. However, the most commonly used MPEG-2profiles, Main Profile at Main Level (MP@ML) and Main Profile at HighLevel (MP@HL), use half resolution horizontally and vertically. MPEG-4versions 1 and 2 use half resolution vertically and horizontally. Notethat full chroma resolution is often called 4:4:4, half chromahorizontal resolution is often called 4:2:2, and half vertical andhorizontal resolution is often called 4:2:0. (It should be noted thatthe 4:x:x nomenclature is flawed in its meaning and derivation, but itis common practice to use it to describe the chroma resolutionrelationship to luminance.)

[0016] The filter which reduces the horizontal and vertical chromaresolution under the various MPEG standards is applied to non-linearvideo signals as transformed into the U and V color representation. Whenthe inverse transformation is applied to recover RGB, the non-linearsignals and the filters interact in such a way as to produce artifactsand problems. These problems can be generalized as “crosstalk” betweenthe Y luminance and the U and V chroma channels, along with spatialaliasing.

[0017] Further information on linear versus non-linear representationsand transformations may be found in “The Use of Logarithmic and DensityUnits for Pixels” by Gary Demos, presented at the October 1990 SMPTEconference, and published in the SMPTE Journal (October 1991, vol. 100,no. 10). See also “An Example Representation for Image Color and DynamicRange which is Scalable, Interoperable, and Extensible” by Gary Demos,presented at the October 1993 SMPTE conference and published in theproceedings and preprints. These papers describe the benefits oflogarithmic and linear spaces at various stages of the image compressionprocessing pipeline, and are hereby incorporated by reference.

[0018] Chroma Sub-Sampling

[0019] The reason for reducing chroma resolution for U and V is that thehuman visual system is less sensitive to changes in U and V than it isto changes in luminance, Y. Since Y is mostly green, and U and V aremostly red, and blue respectively, this can also be described as a humanvisual sensitivity being higher for green than for red and blue.However, although U and V are treated the same in MPEG-1, MPEG-2, andMPEG-4, the human visual system is more sensitive to U (with its redcomponent) than to V (with its blue component).

[0020] This difference in chroma sensitivity is embodied in the 1951NTSC-2 color standard that is used for television. NTSC-2 uses a YIQcolor space, where I and Q are similar to U and V (with slightlydifferent weightings). That is, the I channel primarily represents redminus luminance and the Q channel primarily represents blue minusluminance. In NTSC-2, the luminance is given 4.5 MHz of analogbandwidth, and the I chroma channel is given 1.5 MHz of analogbandwidth. The Q channel, representing the blue-yellow axis, is givenonly 0.5 MHz of analog bandwidth.

[0021] Thus, the NTSC-2 television system allocates three times as muchinformation to the I channel than it does to the Q channel, and threetimes as much information to the Y luminance channel than to the Ichannel. Therefore, the bandwidth ratio between the Y luminance channeland the Q (blue minus luminance) channel is nine. These MPEG YUV andNTSC-2 relationships are summarized in the following table: YUV YUV YUVNTSC- Ratio 4:4:4 4:2:2 4:2:0 2 Red, U, and I pixels to 1:1 2:1 4:1 3:1Y Blue, V, and Q pixels to 1:1 2:1 4:1 9:1 Y

Ratio of Chroma Resolution to Luminance

[0022] Clearly there is a greater difference in treatment between theluminance channel and the U and V channels under the MPEG standards thanthe luminance and I and Q channels in the NTSC-2 standard.

SUMMARY

[0023] The invention is directed to methods, systems, and computerprograms for improving compressed image chroma information.

[0024] More particularly, in one aspect of the invention, a color videoimage may be improved by increasing the red resolution for an RGBrepresentation (or the U resolution for a YUV representation) above theresolution used for blue (or V). Using lower resolution for the bluecolor component means less information needs to be compressed, such asin a motion compensated color video image compression system. Thisaspect of the invention includes a method, system, and computer programfor compressing image chroma information of a color video image in avideo image compression system by selecting a resolution for a red colorcomponent of the color video image that is higher than the resolutionfor a blue color component of the color video image.

[0025] Another aspect of the invention is a technique for reducing thelevel of chroma noise that results from any given value of thequantization parameter (QP) used during compression, thereby improvingimage quality. This is accomplished by utilizing a lower value of QP forthe U (=R−Y) channel than for the Y channel. Similarly, the quality ofthe V (=B−Y) channel may also be improved by utilizing a lower QP valuefor the V channel than for the Y channel.

[0026] Another aspect of the invention is a technique useful when highercompression is required. In this aspect, a positive QP bias is appliedto the QP value for the Y channel for use with either or both of the Uand V chroma channels.

[0027] Another aspect of the invention is use of a logarithmicrepresentation to benefit image coding. Logarithmic coding, whenfeasible, can improve coding efficiency for YUV color spacerepresentations of images originally represented as linear RGB pixelvalues. At other processing steps, a conversion to and from linearrepresentations can be beneficial.

[0028] Another aspect of the invention is a method for improving thevideo characteristics of a color video image in a video compressionsystem, including: selecting a set of image channels to represent thecolor video image, including a luminance channel and n chroma channels,where n is at least three; and compressing the luminance channel and then additional chroma channels to a compressed video image.

[0029] The details of one or more embodiments of the invention are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0030]FIG. 1 is a flowchart showing an illustrative method (which may becomputer implemented) for increasing the resolution for U above theresolution used for V in a YUV color space representation.

[0031]FIG. 2 is a flowchart showing an illustrative method (which may becomputer implemented) for applying a QP bias for chroma channels.

[0032]FIG. 3 is a flowchart showing an illustrative method (which may becomputer implemented) for logarithmic coding of luminance and chromainformation.

[0033]FIG. 4 is a flowchart showing an illustrative method (which may becomputer implemented) for coding additional chroma channels in an imagecompression system.

[0034] Like reference symbols in the various drawings indicate likeelements.

DETAILED DESCRIPTION

[0035] Improved Color Coding Precision

[0036] As the quality of images improves with respect to the attributesof reduced noise, extended dynamic range, and extended color range,human sensitivity to color also increases. In particular, it has beenobserved that red in an RGB representation (or U in a YUVrepresentation) often requires higher precision and clarity than iscommonly used in video compression.

[0037] Unless blue is being used for processing (such as blue-screenspecial effects compositing or image analysis), human sensitivity to theblue-yellow chroma axis, as embodied by either blue or V, is adequatelyaddressed by half resolution sampling horizontally and vertically. Thus,one quarter of the total number of pixels of an image providessufficient quality for representing the blue or V chroma axis. However,unlike blue and V, one-half resolution coding of red and/or U issometimes insufficient in quality with respect to large wide-dynamicrange displays and projectors.

[0038] Thus, an image may be improved by increasing the red resolutionfor an RGB representation (or the U resolution for a YUV representation)above the resolution used for blue (or V). Using lower resolution forthe blue color component means less information needs to be compressed,such as in a motion compensated color video image compression system.

[0039] In accordance with the invention, there are three preferredmethods of maintaining increased red (or U) resolution with respect to adownfiltered blue (or V) resolution:

[0040] 1) Use full resolution for red and/or U;

[0041] 2) Use one-half resolution on only one chroma axis, eithervertically or horizontally, for red and/or U; or

[0042] 3) Use a filtered resolution between full size and one-half, suchas ⅔ or ¾, on one or both chroma axes for red and/or U.

[0043]FIG. 1 is a flowchart showing an illustrative method (which may becomputer implemented) utilizing higher resolution for U than theresolution used for V in a YUV color space representation (a similarmethod may be applied to an RGB color space representation):

[0044] Step 101: In an image compression system utilizing a YUV colorspace representation, downsize filter the V (=B−Y) channel of an inputimage to one-half resolution horizontally, and optionally to one-halfresolution vertically.

[0045] Step 102: Downsize filter the U (=R−Y) channel of the image to aresolution higher than the V (=B−Y) channel, preferably being one of:

[0046] a) full resolution;

[0047] b) between one-half and full resolution horizontally, but fullresolution vertically;

[0048] c) between one-half and full resolution horizontally andvertically;

[0049] d) between one-half and full resolution vertically, but fullresolution horizontally.

[0050] Step 103: Compress the YUV image (having luminance Y and thedownsize filtered U and V chroma information) using an MPEG-likecompression system.

[0051] Step 104: Decompress the images into Y, U, and V channels(usually in a different computer).

[0052] Step 105: Convert the U and V channels to full resolution, usingthe appropriate resolution increase (i.e., the reciprocal of thedownsize filter factor used in Step 101 above for V and Step 102 abovefor U).

[0053] Step 106: Optionally, convert the YUV picture to an RGB image forviewing, analysis, or further processing.

[0054] Differential QP Bias for Chroma

[0055] Co-pending U.S. patent application Ser. No. 09/798,346, entitled“High Precision Encoding and Decoding of Video Images” and assigned tothe assignee of the present invention (which is hereby incorporated byreference), teaches various aspects of the use of the quantizationparameter (QP) during compression. Another aspect of the presentinvention is a technique for reducing the level of chroma noise thatresults from any given value of the quantization parameter (QP) usedduring compression, thereby improving image quality. This isaccomplished by utilizing a lower value of QP for the U (=R−Y) channelthan for the Y channel. Similarly, the quality of V (=B−Y) may also beimproved by utilizing a lower QP value for the V channel than for the Ychannel.

[0056] A simple method of implementing a reduced chroma QP value is tosubtract a constant value from the QP value used for the Y (luminance)channel. Alternatively, a separate constant value (lower than the QPvalue for Y) might be used for each of U and V. For example, “2” mightbe subtracted from the QP value for Y to yield the QP value for U, and“1” might be subtracted for the QP value for Y to yield the QP value forV. Any useful value of the amount to subtract can be used, limited onlyby a minimum value of “1” for the applied QP value.

[0057] This method works for constant QP values (variable bit rate). Italso works as well for variable QP values (e.g., in both constant andvariable bit rate motion compensated compression systems), since theinstantaneous QP value can be biased by subtracting a specifieddifference value from the QP value for Y to yield a QP value for each ofU and V.

[0058] Further, the range of these differential chroma-biased QP valuescan be extended using the extended QP range function or lookup, asdescribed in the “High Precision Encoding and Decoding of Video Images”patent application referenced above.

[0059] It is necessary to signal the U and V bias values from theencoder to the decoder unless a pre-arranged value is used. These can bespecified once, for example, for each session, group of pictures (GOP),frame, or image region.

[0060]FIG. 2 is a flowchart showing an illustrative method (which may becomputer implemented) for applying a QP bias for chroma channels:

[0061] Step 201: In an image compression system, reduce the QP value foreach of the U and V chroma channels by a selected value (which may bedifferent for each channel).

[0062] Step 202: Utilize this reduced QP value for the U and V chromachannel compressions, respectively.

[0063] Step 203: Optionally, if variable QP values are used, ensure thatthe reduced U and V QP value is at least

[0064] Step 204: Unless a pre-set bias is to be used, signal or conveythe QP value reduction amount to the decoder as often as it may change(once at a minimum).

[0065] Step 205: Decompress (usually in a different computer) the signalusing the appropriate QP value for U and V (again ensuring that thereduced QP value is at least “1”).

[0066] Step 206: Optionally, view the decompressed images, or use theimages for additional processing or analysis.

[0067] Another aspect of the invention is a technique useful when highercompression is required. In this aspect, a positive QP bias is appliedto the QP value for the Y channel for use with either or both of the Uand V chroma channels (preferably checking against a QP maximum value ofa compression system, if any). Separate bias can be used for each of theU and V channels. Otherwise, the steps of such an embodiment would besimilar to those shown in FIG. 2.

[0068] Logarithmic Coding of Luminance and Chroma

[0069] The paper entitled “The Use of Logarithmic and Density Units forPixels,” referenced above, describes the benefits of a logarithmicrepresentation for dynamic range. Log representations of a matchingdynamic range are somewhat similar to commonly used video transferfunctions. Even though similar, the logarithmic representation is moreoptimal in extensibility, calibration usage, and in orthogonality ofcolor channels than are the various commonly used video representations.

[0070] Another aspect of the invention is use of a logarithmicrepresentation to benefit image coding. It has been discovered thatlogarithmic coding, when feasible, can improve coding efficiency for YUVcolor space representations of images originally represented as linearRGB pixel values (such as at the sensor of a camera). At otherprocessing steps, a conversion to and from linear representations can bebeneficial.

[0071] As described in the “High Precision Encoding and Decoding ofVideo Images” patent application referenced above, chroma crosstalk withluminance is minimized when:

[0072] Ylog=Log(Wr*R+Wg*G+Wb*B)

[0073] U=Log(R)−Ylog

[0074] V=Log(B)−Ylog

[0075] where Wr, Wg, and Wb are the linear weightings for the red,green, and blue components of luminance, and where R, G, and B representa linear light space. These relationships are useful in applying thisaspect of the invention.

[0076]FIG. 3 is a flowchart showing an illustrative method (which may becomputer implemented) for logarithmic coding of luminance and chromainformation:

[0077] Step 301: In an image compression system, perform the followingtransformation on input (e.g., directly from a video camera) linear R,G, and B pixel values:

[0078] Ylog=Log(Wr*R+Wg*G+Wb*B)

[0079] U=Log(R)−Ylog

[0080] V=Log(B)−Ylog

[0081] where Wr, Wg, and Wb are the linear weightings for the red,green, and blue components of luminance.

[0082] Step 302: Optionally, reduce the resolution of the U and V chromachannels (as described above).

[0083] Step 303: Perform motion-compensated compression on this Y, U,and V representation of the moving image.

[0084] Step 304: Decompress the compressed images to restore Y, U, and Vcolor components of the moving image (usually in a different computer).

[0085] Step 305: If optional Step 302 was applied, reverse theresolution reduction to restore full U and V resolution.

[0086] Step 306: Restore the linear R, G, and B pixel values using thefollowing transformation:

[0087] R=anti-log(Y+U)

[0088] B=anti-log(Y+V)

[0089] G=(anti-log(Y)−Wr*R−Wb*B)/Wg

[0090] Step 307: Optionally, convert to other video RGB representations(alternatively, may be done in lieu of Step 306 rather than in additionto Step 306).

[0091] Additional Chroma Axes

[0092] In extended dynamic range and extended contrast range images, itmay be beneficial to augment visible wavelength channels with additionalchannels of image information, both visible and non-visible.

[0093] The range of colors available from any given set of red, green,and blue primaries does not include all possible visible colors. Thecombining of proportions of red, green, and blue primary colors tocreate other visible colors such as yellow, orange, cyan, and brown, isa property of the human visual system known as the “metamerism”.

[0094] As pointed out in the paper entitled “An Example Representationfor Image Color and Dynamic Range which is Scalable, Interoperable, andExtensible”, referenced above, it is possible to add additional colorprimaries to the three primaries of red, green, and blue. In particular,cyan, magenta, and yellow color primaries help to extend the color gamutbeyond the range available from most common red, green, and blue primaryvalues. Further, violet and ultraviolet (which brightens phosphorescentcolors) can also be conveyed.

[0095] Beyond the visible colors, invisible infrared wavelengths haveproven useful in penetrating clouds and haze, and in seeing in the dark.Ultraviolet wavelengths can also be useful for seeing low-amplitudevisible image details, such as fingerprints and surface coatings.

[0096] Further, even in the visible wavelengths, various materials(e.g., smog and underwater algae) often reduce the amount of contrast ordynamic range of some wavelengths. This is why smog can appear brown,giving a brown tint to all objects in the distance, having reduced theblue contrast and dynamic range. This is also why underwater photographycan appear green, blue-green, or blue, since the red end of the visiblespectrum is reduced in contrast and dynamic range.

[0097] The logarithmic relationships between Y, U, and V, as describedabove, will optimize the coding of color relationships for visiblelight.

[0098] In this aspect of the invention, additional chroma channels areadded to the channels encoding three primary wavelengths, typicallyembodied by RGB or YUV representations. Further, when using a YUV colorspace, it is also possible to change the makeup of the Y (luminance)channel to favor the highest amplitude image signals. Thus, for example,the green visible channel might be coded using its own chroma channel,with luminance moving to other wavelength regions. This concept can beextended to where Y luminance is infrared, with red, green, and blue(and perhaps other visible and non-visible primaries) each having theirown chroma channels.

[0099] In accordance with this aspect of the invention, for each newchroma channel, the following should be determined:

[0100] 1) Should the channel be coded differentially from one or moreother channels (usually from luminance, such as U=R−Y)?

[0101] 2) Should the channel be given full resolution with respect toluminance, or can resolution be reduced without impairing the imagequality for a given intended usage?

[0102] The determination in 1) is based upon the correlation of eachcoded channel with other channels. For example, ultraviolet orfar-infrared wavelength images may be relatively uncorrelated to visiblewavelengths, or to each other. In such a case, these channels might becoded without reference to other channels. However, any visiblewavelengths are highly correlated, and thus can almost always benefitfrom being coded with respect to each other.

[0103] Based upon these determinations, a set of image channels can beselected, usually exceeding (or replacing and exceeding) the threeprimary channels (e.g., YUV). For example, the set of selected imagechannels may comprise a Y′ luminance channel, and n chroma channels,such as a U′ first chroma channel, a V′ second chroma channel, and an X′third chroma channel.

[0104] Using this example, and applying motion compensated compression,the selected value of Y′ would be coded with full resolution, and thevarious other chroma channels (U′, V′, X′) would be differentially orindependently coded. All channels can utilize the same motion vector andmacroblock motion compensation structure as would be used forconventional YUV representations, except that there would be additionalchannels. Each such channel would utilize an appropriate resolution withrespect to Y (as determined in step 2 above). In addition, a QP bias (asdescribed above) can be independently applied to each chroma channel, toensure that the desired compression chroma quality is achieved.

[0105] Even when applied only to visible wavelengths, additional chromachannels can ensure not only extended color range and more accuratecolor, but also allow additional clarity, detail, and noise fidelity tobe applied to such highly visible colors as magenta, orange, yellow, andaqua-cyan. These benefits can be particularly significant forwide-dynamic range and wide-contrast range images.

[0106]FIG. 4 is a flowchart showing an illustrative method (which may becomputer implemented) for coding additional chroma channels in an imagecompression system:

[0107] Step 401: In an image compression system, determine an optimalluminance representation for an image, selected based upon widestdynamic range and highest resolution, including optional non-visiblewavelength image signals.

[0108] Step 402: Determine n additional chroma channels to represent theimage, where n is at least three.

[0109] Step 403: Optionally, for each chroma channel, determine whetherit is beneficial to code differentially with respect to luminance and/orone or more other chroma channels.

[0110] Step 404: Determine the resolution desired for each chromachannel image signal from an input with respect to the luminance imagesignal, such resolution being equal to or less than the resolution ofthe luminance, and optionally apply a resolution reduction.

[0111] Step 405: Compress the Y+n chroma image signals using motioncompensated compression.

[0112] Step 406: Decompress the Y+n chroma images (usually in adifferent computer).

[0113] Step 407: If resolution reduction was applied, restore theoriginal resolutions of the chroma channels.

[0114] Step 408: Combine each chroma channel with its differentialcounterpart, if any, from Step 403 above.

[0115] Step 409: Optionally, perform any of the following:

[0116] a) Convert the chroma channels to a viewing space, such as RGB,or to spaces having more than three primaries, and view as a true-colorimage;

[0117] b) Perform the conversion of a) but view as a false-color image(such as mapping infrared to green);

[0118] c) Use the chroma channels without conversion for processingand/or analysis.

[0119] As another option, each chroma channel may have a biased QP valueapplied (either increasing or decreasing), relative to the QP value usedfor the luminance channel, to achieve a desired level of quality foreach chroma channel (i.e., trading off chroma noise versus higher degreeof compression).

[0120] Implementation

[0121] The invention may be implemented in hardware or software, or acombination of both (e.g., programmable logic arrays). Unless otherwisespecified, the algorithms included as part of the invention are notinherently related to any particular computer or other apparatus. Inparticular, various general purpose machines may be used with programswritten in accordance with the teachings herein, or it may be moreconvenient to construct more specialized apparatus (e.g., integratedcircuits) to perform particular functions. Thus, the invention may beimplemented in one or more computer programs executing on one or moreprogrammable computer systems each comprising at least one processor, atleast one data storage system (including volatile and non-volatilememory and/or storage elements), at least one input device or port, andat least one output device or port. Program code is applied to inputdata to perform the functions described herein and generate outputinformation. The output information is applied to one or more outputdevices, in known fashion.

[0122] Each such program may be implemented in any desired computerlanguage (including machine, assembly, or high level procedural,logical, or object oriented programming languages) to communicate with acomputer system. In any case, the language may be a compiled orinterpreted language.

[0123] Each such computer program is preferably stored on or downloadedto a storage media or device (e.g., solid state memory or media, ormagnetic or optical media) readable by a general or special purposeprogrammable computer, for configuring and operating the computer whenthe storage media or device is read by the computer system to performthe procedures described herein. The inventive system may also beconsidered to be implemented as a computer-readable storage medium,configured with a computer program, where the storage medium soconfigured causes a computer system to operate in a specific andpredefined manner to perform the functions described herein.

[0124] A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, some of the steps described above may be order independent, andthus can be performed in an order different from that described.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A method for compressing image chroma informationof a color video image in a video image compression system, includingselecting a resolution for a red color component of the color videoimage that is higher than the resolution for a blue color component ofthe color video image.
 2. A method for compressing image chromainformation of a color video image in a video image compression system,including: (a) downfiltering a blue color component of the color videoimage to a processed blue color component having a first resolutionalong at least one of the horizontal and vertical image axes of thecolor video image; and (b) filtering a red color component of the colorvideo image to a processed red color component having a secondresolution higher than the first resolution.
 3. The method of claim 2,wherein the second resolution is in the range from 0.5 to 1.0 of thefull resolution of the red color component along at least one of thehorizontal and vertical image axes of the color video image.
 4. Themethod of claim 2, further including compressing at least the processedblue color and red color components to a compressed output image.
 5. Themethod of claim 4, further including decompressing the compressed outputimage to obtain uncompressed processed blue and red color components. 6.The method of claim 5, further including upsize filtering the processedblue and red color components to the full resolution of the color videoimage.
 7. The method of claims 1 or 2, wherein the video imagecompression system is a motion-compensated video image compressionsystem.
 8. A method for reducing chroma noise during compression of acolor video image in a YUV video image compression system utilizing aquantization parameter (QP) during compression, including utilizing afirst QP value for the Y color channel of a color video image, and asecond QP value for at least one of the U and V color channels of thecolor video image, wherein the second QP value is less than the first QPvalue.
 9. The method of claim 8, wherein the second QP value isdetermined by applying a bias value to the first QP value.
 10. Themethod of claim 8, further including compressing the color video image,after application of the first and second QP values, to a compressedoutput image.
 11. The method of claim 10, further includingdecompressing the compressed output image using the first and second QPvalues to obtain an uncompressed video image.
 12. A method for achievinghigher compression during compression of a color video image in a YUVvideo image compression system utilizing a quantization parameter (QP)during compression, including utilizing a first QP value for the Y colorchannel of a color video image, and a second QP value for at least oneof the U and V color channels of the color video image, wherein thesecond QP value is greater than the first QP value.
 13. The method ofclaim 12, wherein the second QP value is determined by applying a biasvalue to the first QP value.
 14. The method of claim 12, furtherincluding compressing the color video image, after application of thefirst and second QP values, to a compressed output image.
 15. The methodof claim 14, further including decompressing the compressed output imageusing the first and second QP values to obtain an uncompressed videoimage.
 16. A method for improving the coding efficiency for a colorspace representation of a video image originally represented as linearRGB pixel values in a video image compression system, includingtransforming the linear RGB pixel values of the video image to alogarithmic representation of luminance and chroma channel information.17. The method of claim 16, wherein transforming includes applying thefollowing equations to obtain a YUV logarithmic representation of thevideo image: Ylog=Log(Wr*R+Wg*G+Wb*B) U chroma channel=Log(R)−Ylog Vchroma channel=Log(B)−Ylog where Wr, Wg, and Wb are linear weightingsfor red, green, and blue components of luminance of the video image. 18.The method of claim 17, further including reducing the resolution of theU and V chroma channels of the YUV logarithmic representation.
 19. Themethod of claim 17, further including compressing the YUV logarithmicrepresentation of the video image to a compressed video image.
 20. Themethod of claim 19, further including decompressing the compressed videoimage to a restored YUV logarithmic representation of the video image.21. The method of claim 20, further including transforming restored YUVlogarithmic representation of the video image to linear RGB pixelvalues.
 22. The method of claim 21, wherein transforming includesapplying the following equations to obtain the linear RGB pixel values:R=anti-log(Y+U) B=anti-log(Y+V) G=(anti-log(Y)−Wr*R−Wb*B)/Wg.
 23. Amethod for improving the video characteristics of a color video image ina video compression system, including: (a) selecting a set of imagechannels to represent the color video image, including a luminancechannel and n chroma channels, where n is at least three; and (b)compressing the luminance channel and the n additional chroma channelsto a compressed video image.
 24. The method of claim 23, wherein atleast one chroma channel represents non-visible wavelengths.
 25. Themethod of claim 23, wherein the luminance channel is the image channelhaving the highest dynamic range and resolution.
 26. The method of claim23, further including coding each chroma channel independently from eachother channel.
 27. The method of claim 23, further including coding eachchroma channel differentially with respect to a selected other channel.28. The method of claim 23, further including reducing the resolution ofat least one chroma channel.
 29. The method of claim 23, furtherincluding applying a quantization parameter (QP) value to at least onechroma channel biased with respect to a QP value applied to theluminance channel.
 30. A computer program, stored on a computer-readablemedium, for compressing image chroma information of a color video imagein a video image compression system, the computer program comprisinginstructions for causing a computer to permit selection of a resolutionfor a red color component of the color video image that is higher thanthe resolution for a blue color component of the color video image. 31.A computer program, stored on a computer-readable medium, forcompressing image chroma information of a color video image in a videoimage compression system, the computer program comprising instructionsfor causing a computer to: (a) downfilter a blue color component of thecolor video image to a processed blue color component having a firstresolution along at least one of the horizontal and vertical image axesof the color video image; and (b) filter a red color component of thecolor video image to a processed red color component having a secondresolution higher than the first resolution.
 32. The computer program ofclaim 31, wherein the second resolution is in the range from 0.5 to 1.0of the full resolution of the red color component along at least one ofthe horizontal and vertical image axes of the color video image.
 33. Thecomputer program of claim 31, further including instructions for causingthe computer to compress at least the processed blue color and red colorcomponents to a compressed output image.
 34. The computer program ofclaim 33, further including instructions for causing a decompressioncomputer to decompress the compressed output image to obtainuncompressed processed blue and red color components.
 35. The computerprogram of claim 34, further including instructions for causing thedecompression computer to upsize filter the processed blue and red colorcomponents to the full resolution of the color video image.
 36. Thecomputer program of claims 30 or 31, wherein the video image compressionsystem is a motion-compensated video image compression system.
 37. Acomputer program, stored on a computer-readable medium, for reducingchroma noise during compression of a color video image in a YUV videoimage compression system utilizing a quantization parameter (QP) duringcompression, the computer program comprising instructions for causing acomputer to utilize a first QP value for the Y color channel of a colorvideo image, and a second QP value for at least one of the U and V colorchannels of the color video image, wherein the second QP value is lessthan the first QP value.
 38. The computer program of claim 37, whereinthe second QP value is determined by applying a bias value to the firstQP value.
 39. The computer program of claim 37, further includinginstructions for causing the computer to compress the color video image,after application of the first and second QP values, to a compressedoutput image.
 40. The computer program of claim 39, further includinginstructions for causing a decompression computer to decompress thecompressed output image using the first and second QP values to obtainan uncompressed video image.
 41. A computer program, stored on acomputer-readable medium, for achieving higher compression duringcompression of a color video image in a YUV video image compressionsystem utilizing a quantization parameter (QP) during compression, thecomputer program comprising instructions for causing a computer toutilize a first QP value for the Y color channel of a color video image,and a second QP value for at least one of the U and V color channels ofthe color video image, wherein the second QP value is greater than thefirst QP value.
 42. The computer program of claim 41, wherein the secondQP value is determined by applying a bias value to the first QP value.43. The computer program of claim 41, further including instructions forcausing the computer to compress the color video image, afterapplication of the first and second QP values, to a compressed outputimage.
 44. The computer program of claim 14, further includinginstructions for causing a decompression computer to decompress thecompressed output image using the first and second QP values to obtainan uncompressed video image.
 45. A computer program, stored on acomputer-readable medium, for improving the coding efficiency for acolor space representation of a video image originally represented aslinear RGB pixel values in a video image compression system, thecomputer program comprising instructions for causing a computer totransform the linear RGB pixel values of the video image to alogarithmic representation of luminance and chroma channel information.46. The computer program of claim 45, wherein the instructions forcausing the computer to transform include instructions for causing thecomputer to apply the following equations to obtain a YUV logarithmicrepresentation of the video image: Ylog=Log(Wr*R+Wg*G+Wb*B) U chromachannel=Log(R)−Ylog V chroma channel=Log(B)−Ylog where Wr, Wg, and Wbare linear weightings for red, green, and blue components of luminanceof the video image.
 47. The computer program of claim 46, furtherincluding instructions for causing the computer to reduce the resolutionof the U and V chroma channels of the YUV logarithmic representation.48. The computer program of claim 46, further including instructions forcausing the computer to compress the YUV logarithmic representation ofthe video image to a compressed video image.
 49. The computer program ofclaim 48, further including instructions for causing a decompressioncomputer to decompress the compressed video image to a restored YUVlogarithmic representation of the video image.
 50. The computer programof claim 49, further including instructions for causing thedecompression computer to transform restored YUV logarithmicrepresentation of the video image to linear RGB pixel values.
 51. Thecomputer program of claim 50, wherein the instructions for causing thecomputer to transform includes instructions for causing the computer toapply the following equations to obtain the linear RGB pixel values:R=anti-log(Y+U) B=anti-log(Y+V) G=(anti-log(Y)−Wr*R−Wb*B)/Wg.
 52. Acomputer program, stored on a computer-readable medium, for improvingthe video characteristics of a color video image in a video compressionsystem, the computer program comprising instructions for causing acomputer to: (a) select a set of image channels to represent the colorvideo image, including a luminance channel and n chroma channels, wheren is at least three; and (b) compress the luminance channel and the nadditional chroma channels to a compressed video image.
 53. The computerprogram of claim 52, wherein at least one chroma channel representsnon-visible wavelengths.
 54. The computer program of claim 52, whereinthe luminance channel is the image channel having the highest dynamicrange and resolution.
 55. The computer program of claim 52, furtherincluding instructions for causing a computer to code each chromachannel independently from each other channel.
 56. The computer programof claim 52, further including instructions for causing a computer tocode each chroma channel differentially with respect to a selected otherchannel.
 57. The computer program of claim 52, further includinginstructions for causing a computer to reduce the resolution of at leastone chroma channel.
 58. The computer program of claim 52, furtherincluding instructions for causing a computer to apply a quantizationparameter (QP) value to at least one chroma channel biased with respectto a QP value applied to the luminance channel.
 59. A system forcompressing image chroma information of a color video image a videoimage compression system, including: (a) means for selecting aresolution for a red color component of the color video image that ishigher than the resolution for a blue color component of the color videoimage; and (b) means for applying the selected resolution to compressthe color video image.
 60. A system for compressing image chromainformation of a color video image in a video image compression system,including means for: (a) downfiltering a blue color component of thecolor video image to a processed blue color component having a firstresolution along at least one of the horizontal and vertical image axesof the color video image; and (b) filtering a red color component of thecolor video image to a processed red color component having a secondresolution higher than the first resolution.
 61. The system of claim 60,wherein the second resolution is in the range from 0.5 to 1.0 of thefull resolution of the red color component along at least one of thehorizontal and vertical image axes of the color video image.
 62. Thesystem of claim 60, further including means for compressing at least theprocessed blue color and red color components to a compressed outputimage.
 63. The system of claim 62, further including means fordecompressing the compressed output image to obtain uncompressedprocessed blue and red color components.
 64. The system of claim 63,further including means for upsize filtering the processed blue and redcolor components to the full resolution of the color video image. 65.The system of claims 59 or 60, wherein the video image compressionsystem is a motion-compensated video image compression system.
 66. Asystem for reducing chroma noise during compression of a color videoimage in a YUV video image compression system utilizing a quantizationparameter (QP) during compression, including (a) means for utilizing afirst QP value for the Y color channel of a color video image, and asecond QP value for at least one of the U and V color channels of thecolor video image, wherein the second QP value is less than the first QPvalue; and (b) means for applying the selected QP values duringcompression of the color video image.
 67. The system of claim 66,wherein the second QP value is determined by applying a bias value tothe first QP value.
 68. The system of claim 66, further including meansfor compressing the color video image, after application of the firstand second QP values, to a compressed output image.
 69. The system ofclaim 68, further including means for decompressing the compressedoutput image using the first and second QP values to obtain anuncompressed video image.
 70. A system for achieving higher compressionduring compression of a color video image in a YUV video imagecompression system utilizing a quantization parameter (QP) duringcompression, including: (a) means for utilizing a first QP value for theY color channel of a color video image, and a second QP value for atleast one of the U and V color channels of the color video image,wherein the second QP value is greater than the first QP value (b) meansfor applying the selected QP values during compression of the colorvideo image.
 71. The system of claim 70, wherein the second QP value isdetermined by applying a bias value to the first QP value.
 72. Thesystem of claim 70, further including means for compressing the colorvideo image, after application of the first and second QP values, to acompressed output image.
 73. The system of claim 72, further includingmeans for decompressing the compressed output image using the first andsecond QP values to obtain an uncompressed video image.
 74. A system forimproving the coding efficiency for a color space representation of avideo image originally represented as linear RGB pixel values in a videoimage compression system, including: (a) means for inputting linear RGBpixel values of a video image; and (b) means for transforming the linearRGB pixel values of the video image to a logarithmic representation ofluminance and chroma channel information.
 75. The system of claim 74,wherein transforming includes applying the following equations to obtaina YUV logarithmic representation of the video image:Ylog=Log(Wr*R+Wg*G+Wb*B) U chroma channel=Log(R)−Ylog V chromachannel=Log(B)−Ylog where Wr, Wg, and Wb are linear weightings for red,green, and blue components of luminance of the video image.
 76. Thesystem of claim 75, further including means for reducing the resolutionof the U and V chroma channels of the YUV logarithmic representation.77. The system of claim 75, further including means for compressing theYUV logarithmic representation of the video image to a compressed videoimage.
 78. The system of claim 77, further including means fordecompressing the compressed video image to a restored YUV logarithmicrepresentation of the video image.
 79. The system of claim 78, furtherincluding means for transforming restored YUV logarithmic representationof the video image to linear RGB pixel values.
 80. The system of claim79, wherein transforming includes applying the following equations toobtain the linear RGB pixel values: R=anti-log(Y+U) B=anti-log(Y+V)G=(anti-log(Y)−Wr*R−Wb*B)/Wg.
 81. A system for improving the videocharacteristics of a color video image in a video compression system,including means for: (a) selecting a set of image channels to representthe color video image, including a luminance channel and n chromachannels, where n is at least three; and (b) compressing the luminancechannel and the n additional chroma channels to a compressed videoimage.
 82. The system of claim 81, wherein at least one chroma channelrepresents non-visible wavelengths.
 83. The system of claim 81, whereinthe luminance channel is the image channel having the highest dynamicrange and resolution.
 84. The system of claim 81, further includingmeans for coding each chroma channel independently from each otherchannel.
 85. The system of claim 81, further including means for codingeach chroma channel differentially with respect to a selected otherchannel.
 86. The system of claim 81, further including means forreducing the resolution of at least one chroma channel.
 87. The systemof claim 81, further including means for applying a quantizationparameter (QP) value to at least one chroma channel biased with respectto a QP value applied to the luminance channel.